\section{Node Mass Flow Variables}\label{node-mass-flow-variables}

The node mass flow variables merit a little more discussion. Six mass flow variables are defined at each node. They are: MassFlowRate, MassFlowRateMin, MassFlowRateMax, MassFlowRateMinAvail, MassFlowRateMaxAvail and MassFlowRateRequest. These variables hold loop mass flow rate information according to the following definitions.

\begin{itemize}
\item
  MassFlowRate -- this node variable holds the simulation mass flow rate for the current timestep. The remaining four variables serve as limits to MassFlowRate.
\item
  MassFlowRateMax, MassFlowRateMin -- These node variables hold the maximum possible and the minimum allowable flow rates for a particular component. As such, they represent the ``hardware limit'' on the flow rate for the component.
\item
  MassFlowRateMaxAvail, MassFlowRateMinAvail -- these node variables represent the \emph{loop} maximum and minimum flow rate for the current configuration of the loop on which the component resides.
\item
  MassFlowRateRequest~ --~ this node variable applies only to plant and holds the desired mass flow rate for the current timestep.~ This stores a record of what flow was desired.
\end{itemize}
